Flash memory is a semiconductor type of non-volatile memory that does not lose its stored data when its power is turned off. Flash memory is both electronically programmable and erasable, thereby enabling it to be reprogrammed even after it has been wired into an electronic system. Flash memory is used in numerous applications including storing microprocessor programs that control the operation of electronic devices such as cell phones and pagers.
Conventional flash memory is typically comprised of an array of memory cells. Information can be stored in a flash memory cell as a charge on a floating gate of a transistor comprising the cell, or as a charge trapped in a dielectric layer of semiconductor material from which the cell is fabricated. Data stored in a flash memory cell is typically read, i.e., sensed, by increasing the control gate voltage of the memory cell on a common word-line, and then sensing whether current is flowing through the memory cell at the bit-line to which the drain terminal of the memory cell is coupled. The circuit used to sense stored data is called a sense amplifier because it amplifies a small change in the bit-line voltage during the first moments of a read into a full logic voltage swing.
Conventional flash memory operates in two stages: a pre-charge stage requiring low impedance so that a previous read can settle quickly and the memory can prepare for the next read, and a sense stage requiring high impedance in order to achieve a high gain during the read in order to access the stored data as quickly as possible. However, conventional sense amplifiers suffer from a significant drawback. Specifically, they typically have a single fixed impedance value unable to provide both the low and high impedance required for optimum performance.
A related U.S. patent application for a Switched Resistor for Sensing Low Current was filed by the applicant Malcolm Smith on Sep. 25, 1998 and has been assigned application Ser. No. 09/160,411.